Split axes

Split axes can be used to build axes containing axis breaks. While split axes
work exactly like bar axes (they even share the same code except for the
painter), their usage differs. A split axes has a tuple as its data value. The
first value of the tuple identifies a sub-axis (for example by a unique
integer) and the second is the value to be passed to the sub-axis. (For a bar
axis, internally this is basically the same, but there the styles take care of
the wrapping of the data into those tuples.)

!! Note: Currently the graph styles are not aware of axis breaks. A generic
notification mechanism for graph styles should be added at a later point.